Galvanic corrosion is a spontaneous electrochemical process in which one metal corrodes preferentially to another when both metals are in electrical contact, in the presence of an electrolyte. This same galvanic reaction is exploited in primary batteries to generate an electrical voltage. The galvanic circuit is an electrochemical process that generates electrical energy as a result of redox (reduction and oxidation) reactions between e.g., different metals or alloys connected by an electrolyte while in surface to surface contact with each other. It occurs when a number of factors take place in conjunction: there have to be different first and second metals; the first and second metals must be in electrical contact; and both the first and second metal must be in the presence of an electrolyte, such as salt water. The most noble of the two metals becomes the cathode, and the less noble (more “active”) metal becomes the anode, and subject to becoming oxidized, and corrosion.
In this application it will be understood that, unless specifically indicated otherwise expressly or by context, the word “metal” or “metals” is meant to include both the pure elemental metals such as Fe, Mg, Au and Cu, as well as metal alloys, such as steels, bronzes, brasses and the like, which may comprise two or more elements or metals. It will be understood herein that the words “noble” and “active” are used in a relative sense in this application unless specifically indicated otherwise. Thus, a metal is more noble than another metal (or alloy) if it is more resistant to corrosion than the other metal. Standard galvanic series of metals are easily available, and provide a listing of different metals and alloys in order (or in reverse order) of their nobility.
As an example, one such listing of a galvanic series is provided herein in the order from more noble to less noble: graphite; gold; platinum; titanium; nickel iron chromium alloy 825; alloy stainless steel; stainless steel-grades 316 & 317; nickel copper alloys—400, k500; stainless steel-grades 302, 304, 321 & 347; silver, nickel 200; nickel chromium alloy 600, nickel aluminum bronze; 70/30 copper nickel; lead; stainless steel-grade 430; 80/20 copper; 90/10 copper nickel, nickel silver; stainless steel-grades 410, 416, silicon bronze; manganese, admiralty brass; aluminum brass; 50/50 lead tin solder; copper; tin; naval brass; yellow brass; red brass; aluminum bronze; austenitic cast iron; low alloy steel; mild steel; cast iron; cadmium; aluminum alloys; beryllium; zinc; magnesium.
Dissimilar metals and alloys have different electrode potentials, and when two or more come into contact in an electrolyte, one metal acts as anode and the other as cathode. The electro potential difference between the dissimilar metals is the driving force for an accelerated attack on the anode member of the galvanic couple. The oxidation of the anode alters its molecular structure, corroding it gradually to the point at which the anode losses structural strength and may become totally disintegrated, gradually dissolving into the electrolyte if the process is permitted to continue unhindered.
Most floating vessels and other marine structures comprise underwater components made of two or more dissimilar metals which may be placed in direct and/or electrical contact with each other, like a bronze propeller attached to a stainless steel shaft, for example, in stern drive or outboard motor-driven vessels. When immersed in an electrolyte, such as sea water, the lesser noble metal, will thus be subject to galvanic corrosion. Over a period of time, this process can deteriorate such metallic underwater parts, thereby potentially causing catastrophic mechanical failures. Such failures may include, for example, the failure of metal parts of the steering system if the rudder, its hinges, pintle (pivot pin) or the gudgeon (fitting for the pintle) is damaged, or of the propulsion system if the propeller's blades edges are corroded. Indeed such galvanic corrosion may even endanger the water-tight integrity of a boat by damage of through-hull fittings, like the engine's cooling system water intakes, or waste water outlets, which can, in extreme cases, lead to valuable property loss and even the chance of human casualties.
Electrolytic corrosion is a non-spontaneous electrochemical reaction, which may occur when stray current finds a path comprising, for example, a vessel's shaft and propeller, such as for instance, from the vessel's direct current (DC) battery bank through a “ground fault”; such as if a positive DC wire comes in contact with bilge water, or surfaces moistened by spray or sea breeze. Alternate current (AC) leaks from, for example, shore power sources may also contribute to this usually aggressive and often unpredicted electrolytic corrosion, sometimes referred to as “electrolysis”, which could result in metal parts being structurally weakened or damaged beyond repair as quickly as in a matter of weeks. Even when a submerged metallic element cast of an alloy such as bronze is not in direct physical contact with any other metallic part, this electrolytic corrosion will still corrode the more active (less noble) element of this alloy; depending on the type of bronze used, the alloy could contain up to 10% of aluminum or zinc (or in the case of a brass part, up to 33% zinc), and if the corroded metal happens to be a through-hull fitting, this could lead to the vessel sinking, with tragic consequences. There are documented cases of vessels sinking by electrochemical corrosion damage with human fatalities included, so the prevention of this problem in floating vessels is not only relevant, as most boat owners believe, based on potential financial losses, but may have a human toll as well.
Commonly, galvanic and electrolytic corrosion is prevented and/or controlled in the marine industry by integrating sacrificial anodes into the natural galvanic circuit, so expensive metallic submerged components are protected by becoming the cathodes, rather than the anodes, in the circuit. Sacrificial anodes are often streamlined-shaped cast lumps of low nobility (highly active) metals in the galvanic series, such as zinc, aluminum and magnesium. Systems into which sacrificial anodes may be incorporated include the following:
Direct Contact Systems
Such systems function by simply attaching one or more sacrificial anodes so that their surfaces directly touch the surfaces of the metal(s) designed to be protected, e.g., by bolting or welding. Direct contact systems are mostly used by steel hull vessels, such as vessels in the fishing industry, commercial cruisers, and some military vessels; this can be expensive because it requires the vessel to be hauled out of the water. In the case of (mostly) smaller, fiberglass, aluminum or wooden-hulled craft, for example, pleasure craft yachts, divers may install and regularly replace, as necessary, specially designed anodes that are fastened directly over parts like shafts, rudders, struts.
Bonding Circuit Systems
A similar but more sophisticated method to control unwanted galvanic and electrolytic corrosion is the implementation of a “bonding system”, electrically linking metallic components together in a single protective circuit. For example, such a system may comprise a highly conductive, low resistance (such as 8 or 6 American wire gauge (AWG)) copper wire which is welded to inboard portions of all the submerged metallic parts of a vessel, like through-hull fittings, struts, shafts, rudders, etc. The bonding system may run between these metallic parts in a loop, or may link one or more such parts through individual wires from the part(s) to a bus bar; that is, a common connection point for the metals to be protected to be connected to the bonding circuit. Such a bonding circuit tends to equalize the redox potentials of the different metallic components, which collectively become the cathode. Both ends of this wire circuit can then be joined to a sacrificial anode, more active than any of the metals of the cathode. The bus bar can thus provide a convenient common attachment point for connected parts to the sacrificial anode via a wire or other conductive connection.
For example, the wire can meet at two threaded rods that go through the hull and hold a fastened sacrificial anode (usually cast of zinc) on the exterior of the vessel under the water line, providing cathodic protection to all metals connected to this bonding semi circuit system. The term “semi circuit” is sometimes used herein to describe the bonding circuit, since the bonding circuit only becomes a full circuit when the vessel is immersed in, and the bonded metallic components are exposed to, an electrolytic fluid medium.
Many modern yachts are built with an integrated bonding circuit system, but again, a boat owner/skipper often only visually inspects the condition of the underwater components of his yacht by himself every other year at most, when he has the boat hauled our of the water. Thus, the owner or skipper usually hires a diver to inspect the boat on a monthly basis and replace the anodes generally whenever this diver considers it convenient.
Hiring a diver is not only an expensive service, but more alarmingly, does not guarantee that the submerged metallic components will be free of electrochemical corrosion. For example, factors including: fraying of wires by vibration and/or heat from the engine; direct contact of the bonding wires with salt water from the bilge; and other short circuits can break or otherwise defeat the bonding circuit. Additionally, other scenarios include the diver's failure to adequately clean the metallic surfaces (for example, stainless steel shafts, or trim tabs) between anode replacements that need to be in clean electrical contact. Such surfaces may sometimes develop an almost imperceptible non-conductive crust of oxidation (such as chromium oxide), by chemical reaction with the oxygen in the water. Another cause of loss of conductivity may be caused by the growth of algae, barnacles, coral tubes (also known as worm tubes); and/or other marine organisms, causing “bio-fouling” which creates resistance to electron flow. Indeed, a simple failure to maintain an electrically tight fit to the fasteners that hold the anode in place, might break the circuit and leave all submerged metallic components vulnerable to electrochemical corrosion.
Hanging Anodes
A type of sacrificial anode (termed a “hanging anode”; sold under the name “Grouper Zinc”, “Mermaid Zinc” and the like, comprises a lump of a galvanic active metal, such as a zinc or magnesium alloy (often cast in the shape of a fish) with a single wire embedded in it, and may comprise an alligator clamp or terminal ring at the opposite end of the cable.
This hanging anode permits a boat owner or operator to connect the alligator clamp or terminal ring directly to whatever metallic component of the vessel he wants to protect, or if the vessel has a bonding system, directly to one of the circuit's bolts on the inside of the vessel, so the owners/operators can monitor how the anode is working by themselves, and without the intervention of a diver, just by pulling the anode out of the water.
However, the hanging anode has not enjoyed widespread acceptance by the boating or marine engineering community for a variety or reasons. Firstly, the hanging anode assembly is often quite unsightly. Often within days after a hanging anode is placed in the water, algae can start growing over the wire connector, and soon will grow over the actual anode. This may soon be followed by mussels and barnacles, creating an unsightly system that is progressively less effective with time. One cannot use an anti-fouling paint coat to coat the anode, since it would reduce or block electrical conductivity between the anode and the electrolyte solution.
Furthermore, the hanging anode may damage the gel coat, deck, topsides, or hull of the boat due to impact, or friction, or gradual wear due to a “pendulum effect” of the heavy, rough metallic anode rubbing against a pristinely buffed gel coat hull, varnished teak surfaces, or painted areas, or when, for example, the hanging anode is removed from the water. Indeed, just the rocking motion of the sea when handling this anode (which could sandwich the hanging anode between the dock or a fender and the boat), may cause damage.
Additionally, the electrical connection between the alligator clip or similar fastener and whatever point in the bonding system it might be attached to, may often provide a rather subpar, weak and unstable electrical contact rather than a firm and secure, point of contact with optimal conductivity.
International Patent Publication No. WO 2007013826 describes a housing for sacrificial anodes for the use of borne vessels in which the anode is always attached to the vessel. In this system, a compartment built into the hull of the vessel is accessible from within the interior of the vessel for changing and monitoring the sacrificial anode; when the watertight access hatch is closed, the anode is directly exposed to the water on the outside of the boat.
British Patent Application No. GB 803,863 describes a system to prolong the life of a sacrificial anode assembly comprising a perforated ion current-limiting assembly surrounding the anode and limiting the current flow from the anode to the cathode. The anode is directly bolted to the cathode; between the anode and cathode is a layer of insulation prevents contact except via the bolts.
British Patent Application No. GB 809,006 describes a system drawn to cathodic protection of ferrous metal in or constituting a tank in a tanker ship. The anode is protected by an insulating cage and is suspended from an insulated, current carrying cable connectable to a source of electric current through a vented shaft-like pipe extending through the top of the tank into the ballast water therein.
U.S. Pat. No. 7,635,237 is drawn to an anode column for protecting a marine structure such as an offshore oil rig from corrosion comprising an elongated guide an elongated conductive anode carrier surrounding the guide and designed to be affixed to a seabed in an upright orientation, at least one sacrificial anode carried by the anode carrier, and an electrical conductor extending from the column and adapted to be connectable to the marine structure.
Without any limitation of the scope of the present invention intended or made, there is clearly a need for preferably portable, preferably handheld, systems that permit the facile installation, monitoring and replacement of sacrificial anodes by the owner or operator of a marine structure (e.g., a boat, dock, or platform). Such systems may ideally obviate the need to place, inspect, replace and service the anodes by diving, at dry dock, or to necessarily apply an auxiliary electrical current as on expensive impressed current cathodic protection systems. Even in cases where traditional sacrificial anode systems are used, the parallel use of such a system would result in lower cost through less frequent replacement of the anodes, and reduced divers' labor charges, while working as an ideal back up in case of a primary system failure.
There is also a need for improved electrochemical corrosion prevention systems in which an anode is contained within a housing that provides a cushion between the anode metal and surrounding marine structure(s), thus protecting such structures from damage resulting from contact with the anode metal. Unlike bare anodes that are simply dangled into the water by a cable with an alligator clip, such a system may utilize an anode which is largely surrounded by a housing that may be pliable, elastic, inflatable and/or which otherwise buffers the marine structure from the metal anode.
There is further a need for improved electrochemical corrosion prevention systems employing sacrificial anodes whereby the anode is preferably contained in a housing that can be treated with an anti-fouling coating, such as anti-fouling paint, to avoid marine growth. This can be particularly advantageous when the housing is positively buoyant in water and is visible in a harbor or marina; the lack of unsightly marine growth provides cosmetic advantages not capable of being provided otherwise.
Additionally, there is a need for portable systems that comprise means for providing a solid, stable electrical contact between metal parts and an anode that can be readily connected (and disconnected) as a quick and easy temporary emergency solution; for example, to protect metal parts against damage from sudden rogue stray current corrosion scenarios in the time frame before diagnosis of the source of the problem and/or necessary repairs are performed by a marine electrician. Furthermore, such systems are particularly advantageous in dangerous conditions that involve, for example, low visibility murky waters caused by extreme weather conditions like heavy rain or strong currents, or in hazardous polluted waters, where having a diver perform inspections and/or take care of anode installations is unfeasible.
Each and every publication, patent and published patent application cited in this patent application is hereby individually incorporated by reference in its entirety as part of this patent application.